In a known piezoelectric ally actuated fuel injector, a piezoelectric actuator is operable to control the position occupied by a control piston, the piston being moveable to control the fuel pressure within a control chamber defined by a surface associated with the valve needle of the injector and a surface of the control piston. The piezoelectric actuator includes a stack of piezoelectric elements, the exercitation level, and hence the axial length, of the stack being controlled by applying a voltage across the stack. Upon de-exercitation of the piezoelectric stack, the axial length of the stack is reduced and the control piston is moved in a direction which causes the volume of the control chamber to be increased, thereby causing fuel pressure within the control chamber to be reduced. The force applied to the valve needle due to fuel pressure in the control chamber is therefore reduced, causing the valve needle to lift away from a valve needle seating so as to permit fuel delivery into the associated engine cylinder.
In order to cause initial movement of the valve needle away from its seating, a relatively large retracting force must be applied to the valve needle. In known piezoelectric ally actuated fuel injectors, the large retracting force applied to the valve needle is maintained throughout opening movement of the valve needle to its full lift position. However, once valve needle movement has been initiated, a reduced force is sufficient to cause continued movement of the valve needle towards its full lift position. Known fuel injectors of this type are therefore relatively inefficient as a significant amount of energy is wasted in applying a large retracting force to the valve needle throughout its full range of movement.
It is an object of the invention to provide a fuel injector which alleviates this problem.